The importance of amino acid residues considered, on the basis of X-ray diffraction studies and conservation of position in glucose-phosphorylating enzymes from yeast to mammalian brain, to be essential for catalytic activity of hexokinases was investigated. A serine residue at position 603 in tumor hexokinase was found to be essential for activity, since more than 90% of wild type activity was lost upon changing it to alanine. A derivative of this ongoing study was the discovery that overexpression of hexokinase or any other protein inserted into a plasmid containing the phosphate-regulated pho A promoter causes induction of a glucose-phosphorylating enzyme distinct from the hexokinase being studied.